Clonidine, but not bradykinin or ANP, inhibits Na+ and water transport in Dahl SS rat CCD.

We examined the actions of potentially natriuretic autacoids in the isolated perfused cortical collecting duct (CCD) dissected from inbred Dahl (Rapp strain) salt-sensitive rats (SS). Atrial natriuretic peptide (ANP, 10 nM), bradykinin (BK, 10 nM), and clonidine (1 microM) were studied to determine their effects on the lumen-to-bath flux of 22Na+ (J1-->b, pmol min-1 mm-1), hydraulic conductivity (Pf, micron/s), and transepithelial voltage (VT, mV). ANP and BK have been shown by others to significantly reduce net Na+ reabsorption and hydraulic conductivity in the Sprague-Dawley (SD) rat CCD, but previous results from our laboratory showed no ANP or BK effect in the SD CCD. In the present study, we were also unable to observe any effect of either ANP or BK in the SS rat CCD. However, in the presence of AVP, clonidine (a partial alpha 2-adrenergic receptor agonist) significantly reduced J1-->b and Pf from 139 +/- 6 (SEM) to 88 +/- 7 and from 959 +/- 176 to 490 +/- 73, respectively. In addition, clonidine significantly depolarized VT from -14.5 +/- 2.8 to -11.2 +/- 1.8. However, unlike its effects in the SD rat CCD, yohimbine (300 nM, an alpha 2-adrenergic receptor antagonist) did not significantly reverse the effects of clonidine on J1-->b, Pf or VT in the SS rat CCD.